fix: resolve frontend code quality issues

- Create missing tsconfig.node.json for Vite config compilation
- Create postcss.config.js for Tailwind CSS processing
- Add test dependencies (vitest, testing-library, jsdom)
- Remove missing /vite.svg favicon reference from index.html

.superpowers/brainstorm/269331-1778521434/state/server-stopped

@@ -0,0 +1 @@
+{"reason":"idle timeout","timestamp":1778524615786}

.superpowers/brainstorm/269331-1778521434/state/server.log

@@ -0,0 +1,6 @@
+{"type":"server-started","port":63865,"host":"127.0.0.1","url_host":"localhost","url":"http://localhost:63865","screen_dir":"/home/alex/projects/backup-tool/.superpowers/brainstorm/269331-1778521434/content","state_dir":"/home/alex/projects/backup-tool/.superpowers/brainstorm/269331-1778521434/state"}
+{"type":"screen-added","file":"/home/alex/projects/backup-tool/.superpowers/brainstorm/269331-1778521434/content/architecture-overview.html"}
+{"type":"screen-added","file":"/home/alex/projects/backup-tool/.superpowers/brainstorm/269331-1778521434/content/data-model.html"}
+{"type":"screen-added","file":"/home/alex/projects/backup-tool/.superpowers/brainstorm/269331-1778521434/content/frontend-views.html"}
+{"type":"screen-added","file":"/home/alex/projects/backup-tool/.superpowers/brainstorm/269331-1778521434/content/backup-flow.html"}
+{"type":"server-stopped","reason":"idle timeout"}

.superpowers/brainstorm/269331-1778521434/state/server.pid

@@ -0,0 +1 @@
+269339